1. Field of the Invention
The invention relates to an exhaust gas-cooling pipe element provided between an exhaust port and an exhaust branch pipe of an internal combustion engine, and to an internal combustion engine exhaust system in which an exhaust gas-cooling pipe element is disposed between an exhaust port and an exhaust branch pipe.
2. Description of the Related Art
A technology of preventing heat damage or adverse heat effect in an exhaust system of an internal combustion engine by disposing a cooling adaptor that corresponds to an exhaust gas-cooling pipe element between the cylinder head and the exhaust manifold is disclosed in, for example, Japanese Utility Model Application Publication No. 64-15718 (JP-U-64-15718) (page 1, and FIGS. 2 to 5).
In the cooling adaptor of JP-U-64-15718, exhaust gas discharged from the exhaust ports of the internal combustion engine passes through the cooling adaptor that has pipe wall surfaces parallel to the flow of the exhaust gas, and then flows into the exhaust branch pipes. Mere passage of high-temperature exhaust gas through the cooing adaptor in this manner is not able to achieve sufficient heat exchange between the exhaust gas and the pipe wall of the cooling adaptor. Therefore, it is necessary to enhance the heat exchange efficiency and therefore enhance the cooling capability.
Furthermore, in the cooling adapter of JP-U-64-15718, when water vapor in exhaust gas condenses on the pipe wall surfaces during a warm-up of the internal combustion engine or the like, there is a possibility of the condensate water gathering in a lower portion of the cooling adaptor due to the gravity and then flowing backward toward the exhaust ports of the internal combustion engine.